You know, I think just about everyone who has used this script has gotten that error. However, it is not a script error, it's just an error in your configuration. I should probably add a note in the script's comments, as well as in the post. You will get that error due to a mistake you made in configuring the class requirement section.
You see, how the configuration works is this.
when CLASS_ID then return [0,1,2,3,4,5,6,7,8...]
Each entry in that array corresponds to how many levels a specified class in the database needs (except the first one, which is a dummy element that the script needs). So, if you have [0,0,0,0,0,0,5] in the array, then you would need 5 level ups with class number 6. In the script's default configuration, there are a total of 19 classes. But a default RMXP project only has 8 classes. Due to how the script checks class levels, you will get an error if there are more entries in the array than you have classes. So, to avoid this error, just make sure you have the configuration set right. You need one more element in the array than you have total number of classes. The first element in the array is a dummy 0, but everything after that corresponds to the class in the database (the 2nd entry is ID number 1, which is fighter in the default configuration, 3rd entry is ID number 2, which is the lance, etc.)
Sorry for the inconvenience, I should probably mention this in the script's configuration section. (sorry if this was confusing)
I'm glad you like the script, BTW.